1. Field of the Invention
The present invention relates to an endoscope objective lens system.
2. Description of the Prior Art
In a conventional endoscope objective lens system, the lens elements thereof are often cemented (hereinafter, cemented lens elements) in order to correct aberrations. However, such cemented lens elements have been costly to manufacture, because centering among very small lens elements has to be done precisely, and thereafter, cementing such small lens elements is further required.
In addition to the above, for the purpose of sterilizing an endoscope, high-pressure steam sterilization (an autoclave) has been frequently used. However, due to steam and a high temperature for sterilization, the layer of cementing agent often comes off, or, the quality of bonding agent deteriorates. Consequently, the image quality of the endoscope becomes worse.
To eliminate such a drawback as mentioned above, an endoscope objective lens system without cemented lens elements has been proposed.
(i) Japanese Unexamined Patent Publication (JUPP) No. H05-341185 has disclosed an endoscope objective lens system constituted by two single lens elements; and
(ii) JUPP No. H05-107470 has disclosed an endoscope objective lens system constituted by three single lens elements.
However, in the above publications (i) and (ii), the correcting of lateral chromatic aberration, that of axial chromatic aberration, and that of field curvature are insufficient; and telecentricity is inadequate.
(iii) JUPP No. H08-334688 has disclosed an endoscope objective lens system which is constituted by four lens elements and achieves a longer back focal distance. However, a first lens element is the only lens element having a negative refractive power (hereinafter, a negative lens element), so that lateral chromatic aberration is undercorrected, and telecentricity becomes worse.
(iv) JUPP No. 2002-228926 has disclosed an endoscope objective lens system which is constituted by four lens elements divided into a front lens group and a rear lens group, and a negative lens element is provided in each of the front and rear lens groups, respectively.
However, the refractive power of the negative lens element of the front lens group is weaker, and the back focal distance is shorter. Moreover, the refractive power of the negative lens element of the rear lens group is also weaker, and the correcting of lateral chromatic aberration is insufficient.
(v) JUPP No. S62-173415 has disclosed an endoscope objective lens system in which the refractive power of a negative lens element of a positive rear lens group is weaker. Consequently, the correcting of lateral chromatic aberration is insufficient.
(vi) JUPP No. H10-20189 has disclosed an endoscope objective lens system constituted by three or four lens elements divided into a front lens group and a rear lens group. However, the rear lens group is constituted by one or two lens elements, so that lateral chromatic aberration largely occurs, and telecentricity becomes worse. While a rear lens group constituted by three or four lens elements has been proposed, the refractive power of a negative lens element of the rear lens group is stronger, and telecentricity becomes worse. Moreover, due to decentration caused by a gap between a lens element and a lens frame upon being assembled, decentration sensitivity (i.e., how much aberrations occur depending on decentration) becomes higher.
(vii) JUPP No. 2004-117607 has disclosed an endoscope objective lens system in which the negative refractive power of a first lens group is stronger than the refractive power of second and third lens groups. Here, with respect to the diaphragm, the first lens group (on the object-side of the diaphragm) can be defined as a front lens group, and the second and third lens groups (on the image-side of the diaphragm) can be defined as a rear lens group. Consequently, telecentricity becomes worse. Moreover, since the radius of curvature of a negative lens element of the rear lens group is smaller, aberrations of higher order occur. Still further, production costs of an endoscope objective lens system becomes higher, since an aspherical lens surface is employed.
As an example of an objective lens system in which cemented lens elements are not employed, an objective lens system of a digital camera, and that of a video camera are known, as disclosed in JUPP No. 2002-303789, JUPP No. 2001-100091, and JUPP No. 2002-277734. However, in the objective lens system disclosed in each of the publications, the negative refractive power of the first lens element (group) is weaker, so that the angle of view thereof is narrower than that of an endoscope objective lens system.
In regard to aberrations, an endoscope objective lens system, particularly, for an electronic endoscope is required to correct lateral chromatic aberration more appropriately than an endoscope objective lens system for a fiber scope does, because of higher pixelation of an imaging device in recent years. Furthermore, a longer back focal distance needs to be secured for positioning a filter group and a cover glass. Still further, in order to prevent shading in the peripheral area of an imaging device, telecentricity of an endoscope objective lens system needs to be improved.